Process for controlling part density in fast injection molding machines

Abstract

A process for controlling part density in molding machines in which a charge of material is injected by a ram into the mold cavity in approximatey 1.5 seconds or less as the ram moves between a variable retraction point and a fixed forward point whereat the ram bottoms, i.e., reaches its forwardmost point of travel beyond which further forward motion is precluded by mechanical limits or stops. Means are included to determine the pressure of the material injected into the mold cavity, using a pressure transducer communicating with the cavity, runner or barrel, at the point in time when the ram bottoms. The measured pressure is then compared against a predetermined pressure correlated to a desired part density, and in response thereto an error signal generated. The error signal is utilized to vary the ram traction point such that the next charge will return the cavity pressure, when the ram tip bottoms, toward the predetermined or desired pressure and in turn return the density of the injected charge toward the desired density notwithstanding changes in density from charge-to-charge.

Having described the invention, what is claimed is: 
     
       1. A method of charging, with a uniform mass of material, a molding cavity having an orifice which communicates, via a mold passage, with an injection barrel in which a ram reciprocates under control of a drive means between a fixed forward position, whereat the ram bottoms with its tip proximate the forward end of the barrel and a variable retracted position, said method comprising: introducing material into said barrel;   retracting said ram to a specified retraction point to provide a specified amount of material between said ram tip and the forward end of said barrel;   advancing said ram in a forward direction through a full stroke until said ram bottoms with its tip proximate the forward end of said barrel to thereby inject into said mold cavity via said mold orifice and passage a charge of material equal in mass to said specified amount, said ram advance being at a speed designed to complete said full stroke in approximately 1.5 seconds or less;   measuring the pressure of said injected material at the end of said stroke when said ram bottoms;   comparing the measured cavity pressure at the end of said stroke against a preset desired cavity pressure correlated to a desired charge of predetermined mass and developing an error signal correlated to the difference therebetween; and   automatically modifying said specified retraction point, and hence the length of said stroke, in response to said error signal to return said measured cavity pressure at the end of said stroke toward said preset desired cavity pressure and in turn restore the successive charge toward said desired charge of predetermined mass notwithstanding variations in material density from charge-to-charge.   
     
     
       2. The method of claim 1 wherein said advancing step is at a speed designed to complete said full stroke in approximately 0.7 seconds or less. 
     
     
       3. The method of claim 1 wherein said introducing step includes introducing material from the class of thermoplastics, thermosets, powdered metals, and/or powdered ceramics. 
     
     
       4. The method of claim 1 wherein said measuring step includes measuring the pressure with a transducer communicating with said cavity. 
     
     
       5. The method of claim 1 wherein said measuring step includes measuring the pressure with a transducer communicating with said mold passage. 
     
     
       6. The method of claim 1 wherein said measuring step includes measuring the pressure with a transducer communicating with said barrel forward of said bottomed screw tip. 
     
     
       7. A method of charging, with a uniform mass of material, a mold cavity having an orifice which communicates, via a mold passage, with an injection barrel in which a ram reciprocates under control of a drive means through a stroke of fixed length between a fixed forward position, whereat the ram bottoms with the forward tip of the ram proximate the forward end of the barrel, and a fixed retracted position, said method comprising: introducing a specified amount of material into said barrel forward of said ram tip;   advancing said ram in a forward direction through a full stroke of fixed length until said ram bottoms with its tip proximate the forward end of said barrel to thereby inject into said mold cavity via said mold orifice and passage a charge of material equal in mass to said specified amount, said ram advance being at a speed designed to complete said full stroke in approximately 1.5 seconds or less;   measuring the pressure in said cavity at the end of said stroke when said ram bottoms;   comparing the measured cavity pressure at the end of said stroke against a preset desired cavity pressure correlated to a desired charge of predetermined mass and developing an error signal correlated to the difference therebetween; and   automatically modifying said specified amount of material introduced into said barrel forward of said ram tip in response to said error signal to return said measured cavity pressure at the end of said fixed length stroke toward said preset desired cavity pressure and in turn restore the successive charge toward said desired charge of predetermined mass notwithstanding variations in material density from charge to charge.   
     
     
       8. The method of claim 7 wherein said advancing step is at a speed designed to complete said full stroke in approximately 0.7 seconds or less. 
     
     
       9. The method of claim 7 wherein said introducing step includes introducing material from the class of thermoplastics, thermosets, powdered metals, and/or powdered ceramics. 
     
     
       10. The method of claim 7 wherein said measuring step includes measuring the pressure with a transducer communicating with said cavity. 
     
     
       11. The method of claim 7 wherein said measuring step includes measuring the pressure with a transducer communicating with said mold passage. 
     
     
       12. The method of claim 7 wherein said measuring step includes measuring the pressure with a transducer communicating with said barrel forward of said bottomed screw tip.